Urinary tract infections are common among women, with one-quarter experiencing recurring infections. And age-related changes increase the likelihood of these infections developing after menopause, when estrogen production plummets.

Until now, taking antibiotics prophylactically -- to ward off recurrent urinary tract infections -- has been the gold standard for these women, said Thomas Hannan, a research instructor in pathology and immunology at Washington University School of Medicine in St. Louis. "But antibiotic resistance is increasing, and some women are resistant to everything we have," Hannan said. "We need other options. We need non-antibiotic options."

This study, published in the June 19 issue of the journal Science Translational Medicine, "suggests a more holistic approach by changing the way women respond to bacteria," said Hannan, co-author of an editorial accompanying the study in the journal.

The results support the use of vaginal estrogen as a preventive measure for postmenopausal women with recurrent urinary tract infections, he wrote in the editorial.

Working in the laboratory and with animal models, the researchers identified a number of ways that estrogen -- the female sex hormone -- helps keep recurrent urinary tract infections at bay.

"This study presents some underlying mechanisms for the beneficial effect of [topical estrogen formulations] after menopause and supports the application of estrogen in postmenopausal women suffering from recurrent UTIs," wrote the study's authors, from the Karolinska Institute in Stockholm, Sweden.

About half of all women will experience at least one urinary tract infection in their lifetime, according to the study. For about 25 percent of these women, the infection will come back again within six months.

Low estrogen levels have previously been linked to recurrent infections, and the new study sought to identify exactly how estrogen might affect a woman's risk of recurrent urinary tract infections.

For the study, the researchers used human cells from postmenopausal women who had used supplemental vaginal estrogen for two weeks. They also worked with mice that were given bacteria that would cause urinary tract infections like those in humans.

They found that estrogen encourages production of natural antimicrobial substances in the bladder. The hormone also makes the urinary tract tissue stronger by closing the gaps between cells that line the bladder. By gluing these gaps together, estrogen makes it harder for bacteria to penetrate the deeper layers of the bladder wall, the study authors said.

Estrogen also helps prevent too many cells from shedding from the top layers of the bladder wall.

"Normally, there's an innate response to infection and some cells die -- sort of taking one for the team -- and then these cells shed," Hannan said. "But shedding too much could allow bacteria to get into the deeper tissue, so this exfoliation is a double-edged sword."

According to the study, estrogen promotes the redistribution of cells and prevents excessive loss of cells during an infection.

One expert welcomed the findings.

"This study gives us more information about what we always had a sense was true: that women do get more [urinary tract infections] after menopause," said Dr. Jennifer Wu, an obstetrician and gynecologist at Lenox Hill Hospital in New York City.

Although taking estrogen in pill form is associated with an increased risk of
breast cancer and cardiovascular disease, topical estrogen is much safer, Wu said. Topical estrogen formulations include creams, gels and vaginal suppositories.

"Only about 1 percent of estrogen cream or other topical delivery gets into the rest of the body's systems," she said.

Although commonplace, women shouldn't accept recurrent urinary tract infections as a normal part of aging, Wu said. "Talk to a gynecologist or urologist to find out what kinds of therapies are available," she said. "There are treatments besides antibiotics."